Using a model to simulate a patient breathing spontaneously through a T-piece from a Puritan nebulizer delivering 44, 74 or 100% oxygen, we evaluated the effects of oxygen flow, ventilatory pattern, and open-ended reservoir (“tail-piece”) volume, on FAO2and FACO2.The FAO2approaches Fio2. with higher oxygen flows into the nebulizer, lower tidal volumes, lower respiratory flow rates, or larger reservoirs. However, with shallow rapid respiration a greater reservoir volume increases FACO2as well. Most importantly, when the nebulizer setting is changed from 40 to 70% or from 70 to 100%, only a negligible increase in FAO2is achieved at an oxygen flow of 5 L/min without the use of a reservoir. We conclude that the T-piece is most safely used with a Puritan nebulizer when the oxygen flow is set at 14L/min (Flush), and the reservoir volume is at least 200 ml.